1. Field of the Invention
The present invention relates to a solar cell module constituted by connecting a plurality of solar cell elements in series or in parallel, particularly to a reliable solar cell module exhibiting excellent conversion efficiency.
2. Related Art
It has recently been predicted that the earth will be warmed due to the greenhouse effect caused by an increase in CO.sub.2 and the like. Therefore, a desire for clean energy from which CO.sub.2 and the like are not produced has arisen.
Nuclear power generation has caused another unsolved problem of radioactive waste, further giving rise to a desire for a source of clean energy.
Solar cells are particularly attractive due to their cleanliness and handling facility.
In particular, amorphous silicon solar cells and polycrystal silicon solar cells have been energetically researched amongst various solar cells because they can be manufactured in a large area at a low cost.
Since, from a practical point of view, a voltage of tens of volts is required, the upper electrodes and the lower electrodes of adjacent solar cell elements must be connected in series.
If shock resistance or flexibility is required, for example, the solar cell substrate may be a conductive one made of stainless steel.
FIGS. 10 and 11 are schematic views which illustrate a conventional series-connected solar cell module constituted by connecting a plurality of solar cell elements by means of wiring members.
Referring to FIGS. 10 and 11, reference numerals 200a and 200b represent conductive substrates, 201a and 201b represent semiconductor layers, 203a and 203b represent upper electrodes, 204a and 204b represent current collecting electrodes, 205a and 205b represent wiring connection members and 206 and 207 represent solar cell elements. Reference numeral 208 represents a solar cell similarly constituted.
The solar cell element 206 is manufactured by sequentially forming the lower electrode 201a, the semiconductor layer 202a, and the upper electrode 203a. Then, the conductive substrate 200a of the solar cell element 206 and the current-collecting electrode 204b of the adjacent solar cell element 207 are electrically connected to each other using the wiring connection member 205a. Furthermore, the adjacent solar cell element 208 and the solar cell element 207 are connected in series.
Since the connection members used to establish the series connection are usually made of metal, a short-circuit may occur because the upper electrode 203a can be separated even if it comes in contact with the connection member 205a. Even if they do not come in contact with each other, another problem arises because the electric current cannot easily flow laterally. Because the lower electrode is made of metal, it is necessary that contact between the aforesaid connection member 205a and the lower electrode 201b of the solar cell element 207 be prevented. Furthermore, contact between, for example, the lower electrodes 201a and 201b of the adjacent solar cell elements 206 and 207 must be prevented. Therefore, the distance between the solar cell elements 206 and 207 must be lengthened. However, this causes the problem that the effective area of the generating region of the solar cell module decreases and the conversion efficiency deteriorates.
If the solar cell module is accidentally bent during use, stress inevitably acts on the connection members disposed between the adjacent solar cell elements, causing the solar cell elements and the series connection members to be short-circuited. As a result, the quality deteriorates and, when the solar cell module is repeatedly bent, the connection member breaks if a thin connection member is used or if the connection member is connected to only a portion of the solar cell.
What is worse, a portion of the connection member used to establish the series connection and appearing on the surface of the solar cell module covers the light-receiving surface of the solar cell element, causing the aforesaid portion to be a non-generating region. In order to reduce the area of the non-generating region, a comb-shaped connection member 210 arranged as shown in FIG. 12 may be employed in place of the connection members 205a and 205b to establish the series connection. However, if the comb-shaped connection member has poor strength, the connection member is broken if the solar cell module having the solar cell elements connected in series by using the aforesaid connection member is repeatedly bent.
If a thick connection member is employed in order to prevent the aforesaid problem, another problem arises in that a thick connection member made of metal such as copper is too strong and causes damage to the surface of the solar cell element at the metal edge portion thereof.
Where the solar cells are connected in series, breakage of the solar cells due to application of a reverse bias voltage must be prevented. If light incident on one cell element of a solar cell module constituted of four solar cell elements connected in series is shielded, the solar cell element cannot generate photovoltaic force, thereby causing the sum of the output voltages of the other solar cell elements to be supplied to the shielded solar cell, as a reverse bias voltage. Therefore, there is a risk that the solar cell element can be electrically destroyed.
In order to prevent application of the reverse bias voltage, a reverse bias prevention bypass diode must be disposed parallel to each solar cell element.
FIG. 13 is a schematic view which illustrates a conventional structure constituted by connecting three solar cell elements in series and reverse bias voltage prevention bypass diode 230 is connected to the upper and lower electrodes of each of the solar cell elements. FIG. 14 is a cross-sectional view taken along line X--X' of FIG. 13.
Referring to FIGS. 13 and 15, reference numerals 240, 241, and 242 represent solar cell elements, each of which is constituted by sequentially forming lower electrode 251, semiconductor layer 252, and upper electrode 253 on conductive substrate 250.
Each of the solar cell elements is constituted by forming comb-shaped electrode 214 as a current-collecting electrode on the upper electrode 253, with bus bar 215, serving as the current-collecting electrode for the comb-shaped electrode 214, placed on the comb-shaped electrode 214. The comb-shaped electrode 214 and the bus bar 215 are electrically connected to each other by conductive adhesive agent 216 so that an output terminal from the upper electrode 253 is formed.
The electrical output can be obtained by connecting portion 220 of the conductive substrate of the solar element 240 to conductive member 219 made of copper, for example, by spot welding or the like.
Then, the bus bar 215 of the solar cell element 240 and the conductive member 219 connected to the conductive substrate of the adjacent solar cell element 241 are connected to each other, so that a series connection is established. Then, a reverse bias voltage prevention bypass diode is disposed between the bus bar 215 of each of the solar cell elements and the conductive member 219 connected to the conductive substrate as by soldering.
However, the aforesaid arrangement wherein a reverse bias voltage prevention bypass diode is connected to each of the solar cell elements requires the reverse bias voltage prevention bypass diode to be positioned apart from each solar cell element. As a result, the ratio of the effective generating area to the overall area of the solar cell module decreases.
The reverse bias voltage prevention bypass diode of ordinary type is molded with resin and has a lead wire (a leg) for connection thereto by soldering, which may result in wire breakage. Therefore, the reverse bias voltage prevention bypass diode must be of a relatively large size, resulting in a portion of the reverse bias voltage prevention bypass diode projecting over the surface of the solar cell element when an encapsulant such as an EVA (ethylene-vinyl acetate copolymer) encloses the solar cell element in the ensuing manufacturing process. As a result, the flatness of the solar cell module deteriorates, and bubbles are easily left at positions adjacent to the reverse bias voltage prevention bypass diode. The laminated member is then easily separated from the solar cell element starting from the bubble portion during outdoor use of the solar cell module.
Even more detrimental, a large quantity of the encapsulant must be used in order to improve the flatness of the solar cell module, and therefore, there is no reduction possible in the overall cost.
The aforesaid conventional solar cell module cannot easily be manufactured and, therefore, the manufacturing process cannot be automated.